1. Field of the Invention
The present invention relates generally to antennas and, more particularly to a method for reducing cross-polar degradation in array-feed dual offset reflector antennas.
2. Description of Related Art
Long distance communications and high-resolution radar applications require antennas having high gain. Reflector-type antenna systems are the most common and widely used high gain antennas. Reflector antennas operating at microwave frequencies routinely achieve gains in excess of 30 dB.
Many applications, such as satellite spot beam coverage of specific geographic areas, require the use of multiple beams from a single reflector antenna. The need for multiple beams is especially pronounced in the Ka band of operation. Ka frequency band signals, such as those from satellite transmitters, are highly attenuated by propagation and atmospheric effects and, therefore, require high gain spot beams to adequately cover required geographic areas.
Synthesis of multiple beams using a single reflector antenna requires the use of dual polarization reflector antennas. Dual polarization reflector antennas can be implemented using dual gridded reflectors or multiple reflectors. Dual gridded reflectors use two orthogonally polarized reflector surfaces that are fed individually by a single feed or an array of feeds. The two reflector surfaces may be parabolic or specially shaped. Each polarization grid is designed to only reflect one polarization of electromagnetic energy. Therefore, the polarization purity of the radiation pattern produced by the antenna is achieved through the use of two polarization grids.
Dual reflector systems utilize a main reflector and a subreflector. Two common configurations of dual reflector antennas are known as "Gregorian" and "Cassegrain." Typically, the main reflector is specially shaped or parabolic and the subreflector is ellipsoid in shape for a Gregorian configuration or hyperboloid in shape for a Cassegrain configuration. In dual reflector systems neither reflector is polarized and, therefore, reflects all polarizations of electromagnetic energy.
When two different polarizations are used on a dual reflector system, cross polarization performance of the system is very important. Optimum cross polarization performance may be achieved through the "Mitzuguchi condition," which is a relationship that governs the location of an antenna feed with respect to the main reflector and the subreflector focal axes. However, the "Mitzuguchi condition" pertains only to the antenna feed at the focus of the reflector system. It is common to feed a reflector system with an array of feeds, only one of which can be in the focus of the reflector system. That is, the feed located in the focus of the system will have optimum cross polarization performance, but off-focus feeds will suffer degraded cross polarization performance.
Referring now to FIG. 1a, a Gregorian dual reflector antenna 10 is shown. The Gregorian dual reflector antenna 10 includes a reflector 14, a subreflector 18, and a feed array 22. The feed array 22, which includes a number of feeds, irradiates the subreflector 18 with electromagnetic energy. The electromagnetic energy is, in turn, transferred from the subreflector 18 to the reflector 14 and radiated to a target from the reflector 14. In the receive situation, electromagnetic energy incident on the reflector 14 is reflected to the subreflector 18. The subreflector 18, in turn, irradiates the feed array, which may be used to convert the electromagnetic energy into voltage for processing by external circuitry (not shown). FIG. 1b represents a Cassegrain dual reflector antenna 11, which also includes a reflector 14, a subreflector 18, and a feed array 22.
Spatial relations in a dual reflector system are made with respect to a Cartesian coordinate system having right-handed reference axes and an origin. The origin represents a reference location in the dual reflector system where x, y, and z are all equal to zero. In the Gregorian dual reflector antenna 10 shown in FIG. 1a, the origin of the reference axes of the right-handed coordinate system is located at the feed array in the focus point of the subreflector 18. The z-axis points directly from the origin to the bisector of the subreflector 18. The x-axis, which is at a 90.degree. angle to the z-axis, is oriented as shown in FIG. 1a. The positive y-axis points from the origin directly into the plane of the paper, which is defined by the x-z plane. The x-y plane bisects the subreflector 18 into first and second portions of equal size. Similarly, the y-z plane bisects the subreflector into third and fourth portions.
FIG. 2 is a diagram illustrating a feed array 22 that may be used to feed the subreflector 18. The feed array 22 includes a plurality of individual feeds 30. While the feed array shown in FIG. 3 includes twenty-five individual feeds 30, the size of the feed array 22 is limited only by the physical constraints of the application. Therefore, some feed arrays 22 may include relatively few individual feeds 30, and some feed arrays 22 may include hundreds or even thousands of feeds 30. A center feed 35 of the feed array 22 is located in the origin of the coordinate system as shown in FIGS. 1 and 2.
FIG. 3 is a diagram of a feed array 22' illustrating nine individual feeds 30 numbered 1-9 that are used to feed the subreflector 18 of the Gregorian dual reflector system 10. The axes of the graph indicate azimuth and elevation of the feeds with respect to the focus of the reflector system. Again, as in FIG. 2 the center feed 35 (feed three) is located directly in the center of the focus and the remaining individual feeds 30 are off-focus as shown. All of the feeds 30, 35 of the feed array 22' are oriented in the same direction. That is, none of the individual feeds 30 shown in FIG. 3 are rotated either clockwise or counterclockwise in the x-y plane. The configuration shown in FIG. 3 is merely exemplary of the types of feed arrays that may be used in conjunction with a reflector antenna system.
FIG. 4 is a plot of the co-polarization performance of the feed array 22' shown in FIG. 3. The co-polarization performance of the feed array 22' is approximately uniform for each of the nine individual feeds 30.
FIG. 5 is a plot of the cross polarization performance of the Gregorian antenna system with the feed structure shown in FIG. 3 and the co-polarization performance shown in FIG. 4. The center feed 35 (feed three) is located in the focus of the reflector system and, therefore, has the best cross polarization performance at -0.37. Conversely, feeds one and five, which are located farthest from the focus, have cross polarization level approximately 20 dB higher than feed three. The feeds 30 farthest from feed three along the y-axis, which is in the focus of the subreflector, have the poorest cross polarization performance. As feeds 30 are positioned closer to feed three along the y-axis, their cross polarization performance improves. Although the results shown in FIG. 4 are for the feed array 22' having nine feeds, the trend of poor cross polarization performance for off-focus feeds is found in every antenna feed configuration.
Because of the need for high gain and multiple beam systems, reflector antennas that are fed with an array of feeds are desirable. However, it can be appreciated that the cross polarization performance of an array fed system is crucial to optimal system performance. Therefore, the need for a reflector system that can be fed with a feed array and has good cross polarization performance can readily be appreciated.